Recently had some truly nasty brown stain to clean off a hull - boat had been in semi freshwater for a long long time.

After Googling around I tried Bar Keepers Friend ($2 at the local Dollar General), which is a powder with Oxalic Acid in it. Worked very well. I just applied it to a wet surface and lightly rubbed with a ScotchBrite backed sponge (using the rough side on the worst spots), then rinsed after about a minute. It took a couple passes around the waterline in short sections, but worked good..